THEIR DAILY BREAD. There are a number of theories about why Russians, upon converting to Christianity in the 10th century, chose the Byzantine Orthodox Church over Roman Catholicism. One theory suggests it all came down to bread. Orthodoxy permitted the use of leavened bread in Eucharist rites; Catholicism did not. As Russia's bread-baking culture developed through the years, many foreigners found the taste -- particularly that of the rye bread -- unpalatably sour. But for Russians, it was an essential and irreplaceable basic. If in Europe the primary sources of protein were traditionally cheese and fish, in Russia it was bread. A poor wheat or rye harvest automatically signified hunger, even though the country's lakes and rivers teemed with fish, and its forests flourished with animals, mushrooms, and berries. Years ago, Russians baked their traditional pirogi with a filling made from flour -- in other words, bread stuffed with more bread. And there's no other place in the world where people eat bread together with other starchy staples, like grains or noodles. Russians eat three times more bread than people in the United States or the European Union. It's clear that bread carries a strong symbolic meaning. To this day, esteemed guests are greeted with an offering of bread and salt. A synonym for gosteprimstvo, or hospitality, is actually khlebosolstvo -- literally, "bread-and-saltiness." Many Russians are reluctant to leave even a crumb of bread uneaten -- perhaps because popular wisdom holds that any bread that's thrown away will be picked up and eaten by the Devil.